The so-called special nuclear materials refer to uranium and plutonium that can be used for the manufacture of nuclear weapons. It is generally required that the abundance of the uranium 235 in uranium and the plutonium 239 in plutonium is at least 93%. Detection of special nuclear materials is detection of existence of uranium 235 and plutonium 239 concealed in an enclosed environment (for example a vehicle or container). Therefore, the special nuclear materials to be detected hereinafter refer to uranium 235 and plutonium 239.
It is known that several kilograms to tens of kilograms of the special nuclear materials are sufficient for constructing a simple nuclear bomb, thereby posing a great threat to social security. In some applications, the detection limit of special nuclear materials is defined as 100 cm3, which is very small compared with the enclosed environment (for example a container) where special nuclear materials are possibly located. In addition, if the terrorists shield and camouflage the special nuclear materials to some extent, the detection difficulty thereof will be further increased. Therefore, how to detect these special nuclear materials that are not in “large quantity” from the goods imported via various ports raises great challenge to the detection technology.
Current detection technology for special nuclear material is usually classified as passive detection technology and active detection technology.
The passive detection technology makes use of the spontaneous disintegration phenomenon of the special nuclear materials. When the atomic nucleus of the special nuclear materials undergoes a spontaneous disintegration, it will release prompt neutrons and γ-ray signals. It is possible to find out the special nuclear materials by collecting/detecting these ray signals by using a detector.
However, the intensity of the signals emitted when the special nuclear materials disintegrates spontaneously is weaker. Therefore, the passive detection result is subject to the magnitude and shielding condition of the special nuclear materials and is very easy to be interfered by the shielding. If what measured by the detector is signal counting not energy distribution, it is impossible to differentiate the signal emitted from the special nuclear material from that comes from the radiation background of the nature (for example γ-ray of potassium 40 and neutrons produced by the cosmic rays). Therefore, the accuracy rate of detection is very low. Besides, the passive detection technology needs longer time to collect spontaneous disintegration signals and is not suitable for the occasion that needs higher detection speed, for example airport or harbour.
Nuclear resonance fluorescence technology is an active detection technology, which makes use of electromagnetic radiation of specific energy to irradiate the special nuclear materials. When the specific energy and the atomic nucleus of the special nuclear materials have the same energy level, strong absorption will occur. It is possible to carry out a complete definitive detection of the existence of the special nuclear materials by the detecting absorption condition of electromagnetic radiation of the specific energy or detecting the energy of γ photons emitted from the object after it absorbs electromagnetic radiation. This is a method having very good accuracy.
However, the nuclear resonance fluorescence technology needs to use a dedicated accelerator to produce monoenergetic, high-energy X-rays. In order to produce monoenergetic X-rays of the magnitude of MeV to irradiate the object to be detected, it needs an electron accelerator more than 100 MeV and high power laser source. At present, this kind of ray source is still in research phase and is not mature enough. Another ray source is to directly make use of braking radiation source, and then the requirement on the electron accelerator is not high. It only needs to accelerate the electrons to the magnitude of MeV to 10 MeV. However, the measurement of the nuclear resonance fluorescence photons at this moment is certainly accompanied with a lot background interference, which brings about great interference to the measurement and is adverse.
In a word, there appears no effective technology at present to detect special nuclear materials (in particular special nuclear materials concealed in an enclosed environment).
It is known that fissile materials, such as the special nuclear materials (namely uranium 235 and plutonium 239) and other nuclear materials (such as uranium 238 and plutonium 240) will undergo a photofission under irradiation of X-rays. Further, the special nuclear materials will undergo a thermal neutron induced fission under the irradiation of thermal neutrons.
Very obviously, it is impossible to determine the special nuclear materials only by using photofission because it will be interfered by other fissile materials such as uranium 238 and plutonium 240. The photofission process cannot differentiate uranium 235 from uranium 238, nor can differentiate plutonium 239 from plutonium 240.
It is conceived to detect special nuclear materials only by thermal neutron induced fission. However, the yield of the fission production produced by the thermal neutron induced fission is smaller, which reduces the sensitivity of detection of the special nuclear materials and is easy to result in false alarm. If thermal neutrons are used to irradiate the special nuclear materials for a long time to accumulate sufficient fission signals, it will result in excessive long detection time, which is adverse in the occasions (such as Customs and harbour) that have requirement on detection speed. Besides, the thermal neutrons are hard to be collimated to a narrow region. Therefore, even if special nuclear materials are found in the object to be detected, it is hard to determine the position thereof.